Effects of zotepine on extracellular levels of monoamine, GABA and glutamate in rat prefrontal cortex

Abstract

Background and purpose: The atypical antipsychotic drug, zotepine, is effective in treatment of schizophrenia and acute mania, but the incidence of seizures during treatment is higher than with other antipsychotics. In addition, the mechanisms underlying the clinical actions of zotepine remain uncharacterized.

Experimental approach: The effects of intraperitoneal administration of zotepine and haloperidol on the extracellular levels of noradrenaline, dopamine, 5-HT, GABA, and glutamate in the medial prefrontal cortex (mPFC) were compared. Neuronal activities induced by each drug in the ventral tegmental area (VTA), locus coeruleus (LC), dorsal raphe nucleus (DRN) and mediodorsal thalamic nucleus (MTN) were also analysed.

Key results: Haloperidol did not affect extracellular neurotransmitter levels in the mPFC. In contrast, zotepine activated neuronal activities in all nuclei and increased the extracellular levels of noradrenaline, dopamine, GABA, and glutamate in the mPFC, but not 5-HT levels. The zotepine-stimulated neuronal activity in the VTA, LC, DRN and MTN enhanced the release of dopamine, noradrenaline, 5-HT, glutamate and GABA in the mPFC, although the enhanced GABAergic transmission possibly inhibited noradrenaline, dopamine and 5-HT release. The other afferent to mPFC, which releases dopamine and noradrenaline, was partially insensitive to GABAergic inhibition, but possibly received stimulatory AMPA/glutamatergic regulation from the MTN.

Conclusions and implications: Our results indicated that the positive interaction between prefrontal catecholaminergic transmission and AMPA/glutamatergic transmission from MTN might explain the regulatory effects of zotepine on neurotransmitter release. A mechanism is suggested to account for the pharmacological profile of this atypical antipsychotic and for its pro-convulsive action.

Figure 1

Typical chromatograms of monoamine analyses using ion-exchange high-performance liquid chromatography equipped with an electrochemical detector and amino acid analyses using extreme high-pressure liquid chromatography equipped with a fluorescence detector. The chromatograms in A were obtained from 20 µL of a standard solution containing 5 fmol (20 µL)⁻¹ of noradrenaline, dopamine, and 5-HT (standard) or prefrontal perfusate (mPFC). The quantification limits for noradrenaline, dopamine and 5-HT were 0.1, 0.2 and 0.5 fmol (20 µL)⁻¹ respectively. The chromatograms in B were obtained from 5 µL of a standard solution containing 1 pmol 5 µL⁻¹ glutamate and 0.2 pmol 5 µL⁻¹ GABA (standard), or prefrontal perfusate (mPFC). The quantification limits for glutamate and GABA were 10 and 20 fmol 5 µL⁻¹ respectively.

Figure 3

Effects of antipsychotic drugs on the extracellular levels of GABA (A) and glutamate (B) in the mPFC. Arrows indicate systemic administration of haloperidol (HPD: 1 mg·kg⁻¹, i.p.) and zotepine (ZTP: 1 and 3 mg·kg⁻¹, i.p.). The ordinates represent the mean ± SD (n = 8) neurotransmitter release (pmol per sample). Effects of the antipsychotic agents were compared using repeated two-way anova with Tukey's multiple comparison (*P < 0.05; **P < 0.01).

Figure 2

Effects of antipsychotic drugs on the extracellular levels of noradrenaline (A), dopamine (B), 5-HT and (C) GABA in the mPFC. Arrows indicate systemic administration of haloperidol (HPD: 1 mg·kg⁻¹, i.p.) and zotepine (ZTP: 1 and 3 mg·kg⁻¹, i.p.). The ordinates represent the mean ± SD (n = 8) neurotransmitter release (fmol per sample). Effects of the antipsychotic agents were compared using repeated two-way anova with Tukey's multiple comparison (*P < 0.05; **P < 0.01).

Figure 4

Typical histograms showing the effects of (A) zotepine (3 mg·kg⁻¹ zotepine, i.p.) and (B) haloperidol (1 mg·kg⁻¹ haloperidol, i.p.) on neuronal firing frequencies in the VTA. Arrows indicate systemic administration of zotepine (ZTP: 3 mg·kg⁻¹, i.p.) and haloperidol (HPD: 1 mg·kg⁻¹, i.p.). The ordinates represent the mean neuronal firing frequencies (spikes sec⁻¹: Hz). Dose-dependent effects of the antipsychotic agents on neuronal firing frequencies in the (C) VTA, (D) DRN, (E) LC and (F) MTN. The ordinates represent the mean neuronal firing frequencies during 60–180 min after administration of haloperidol or zotepine. Low-dose: HPD at 0.1 mg·kg⁻¹ and ZTP at 1 mg·kg⁻¹; high-dose: HPD at 1 mg·kg⁻¹ and ZTP at 3 mg·kg⁻¹. The dose-dependent effects were compared using one-way anova with Dunnett's multiple comparison (*P < 0.05; **P < 0.01).

Figure 5

Effects of glutamate receptor antagonists (MK-801 and DNQX) and a GABA_A receptor agonist (muscimol) on zotepine-induced elevation of extracellular levels of (A) noradrenaline, (B) dopamine and (C) 5-HT in the mPFC. The ordinates represent the mean ± SD (n = 8) extracellular neurotransmitter levels (fmol per sample) during 60–180 min after zotepine administration (ZTP: 3 mg·kg⁻¹, i.p.). The perfusion medium was switched from MRS to MRS without (non-treatment) or with 50 µmol·L⁻¹ MK-801, 50 µmol·L⁻¹ muscimol or 50 µmol·L⁻¹ DNQX. Following stabilization, the rats were injected with either the vehicle (control) or zotepine (ZTP: 3 mg·kg⁻¹, i.p.). Data were compared using two-way anova with Tukey's multiple comparison (*P < 0.05; **P < 0.01 vs. non-treatment, #P < 0.05; ##P < 0.01 vs. control).

Figure 6

Effects of glutamate receptor antagonists (MK-801 and DNQX) and a GABA_A receptor agonist (muscimol) on zotepine-induced elevation of extracellular levels of (A) GABA and (B) glutamate in the mPFC. The ordinates represent the mean ± SD (n = 8) extracellular neurotransmitter levels (pmol per sample) during 60–180 min after zotepine administration (ZTP: 3 mg·kg⁻¹, i.p.). The perfusion medium was switched from MRS to MRS without (non-treatment) or with 50 µmol·L⁻¹ MK-801, 50 µmol·L⁻¹ muscimol or 50 µmol·L⁻¹ DNQX. Following stabilization, the rats were injected with either the vehicle (control) or zotepine (ZTP: 3 mg·kg⁻¹, i.p.). Data were compared using two-way anova with Tukey's multiple comparison (*P < 0.05; **P < 0.01 vs. non-treatment, ##P < 0.01 vs. control).